The present invention relates generally to process control lines such as paper manufacturing equipment and is particularly directed to paper machines of the type which transport webs of paper fibers that are being formed into rolls of finished paper. The invention is specifically disclosed as a method for real time correction of measurements of process variables which, without correction, are manifested as cross direction and machine direction variability in the paper web due to rotating or periodic equipment by use of a time synchronized measurement correction technique.
Computerized monitoring systems have been used to monitor the properties of a moving web of material such as that produced in paper mills. In such systems, the process variables that are monitored relate to the xe2x80x9cmachine directionxe2x80x9d (MD) or the xe2x80x9ccross directionxe2x80x9d (CD), and some process variables are related to physical parameters that are affected in both directions.
One of the problems in the current state of control technology is the limited ability to separate true cross direction variability (which is positionally dependent) from both machine direction variability (which is time dependent) that occurs uniformly across an entire moving web of material, and the effects of rotating equipment which produce machine direction variability that is positionally dependent. One current solution to this limitation is the use of filtering, however, this slows the speed of response of CD control, and sometimes cannot adequately resolve the true profile shape of the web.
The above filtering solution that is currently in use has a limitation because the filtering slows the speed of response of the CD control system. Moreover, the filtered results are not always able to adequately resolve the true profile shape. This is caused by inadequate sampling of the impact of the rotating or periodic equipment, and in certain cases the existence of a synchronization pattern between this equipment and the scanning measurement. A near exact synchronization produces an effect that can be referred to as an xe2x80x9caliased profile,xe2x80x9d in which the alias forms a false profile that is due to sampling the equipment repeatedly at the same position in its periodic cycle.
A breakthrough occurred with the ability to acquire on-line measurements and generate a time-synchronous array map of the moving web while in production, and this array or map can be used to assess the impact of the periodicity of the machine elements. This invention is disclosed in U.S. Pat. No. 5,960,374, titled xe2x80x9cSystem for Time Synchronous Monitoring of Product Quality Variable,xe2x80x9d issued to Lausier on Feb. 14, 1997. The Lausier system builds count maps and measurement maps from a predetermined number of rotating elements. A scanning measurement frame is mounted across the width of the moving product web and takes on-line measurements of a selected variable in the cross direction and machine direction of the web. Event trigger signals from the sensors are coupled to elements of the process line, and the CD and MD measurements from the scanning sensors in a particular measurement frame are provided to the computer system. Data measurement xe2x80x9cboxesxe2x80x9d are used to receive the scanned measurements of paper quality variables (in the case of a paper mill) over the surface of the web, in which there are spatial CD zone increments and MD position/slash time increments that define the data measurement boxes. Given a sufficient amount of time and samples taken, the effect of all mapped rotating elements can be determined.
An example of the synchronization pattern referred to above is illustrated in FIG. 5, which depicts the count matrix comprising the number of scanned measurements that occurred when the coating rod was at each rotational position and the scanner was at each cross directional position. With perfect sampling, one would expect the same number of counts in each measurement cell. However, as can be seen on FIG. 5, some of the measurement cells are not sampled at all, while others were sampled over 180 times during the time interval during which samples were taken. Consequently, the value of the measurement cells in the heavily sampled regions will dominate the calculated values, and will falsely bias the profile estimation. This sampling error is not random, but appears in diagonal or cross-hatched patterns. In one process equipment installation, the cross-hatched pattern was produced by an almost exact 10:1 ratio between the scanner pattern and rod rotation period.
Accordingly, it is a primary advantage of the present invention to correct the measurement profile of a processing line of periodically-moveable equipment, such as rotating equipment, with little or no filtering. It is another advantage of the present invention to remove aliasing effects in substantially real time while measurements are being collected for a process line utilizing rotating equipment, such as used in a paper mill. It is a further advantage of the present invention to create accurate profiles of rotating equipment process lines that can be used for cross machine control with little or no filtering to provide immediate correction for rotational effects at the time of sampling and before the data is averaged or used in other ways, thereby allowing the control system to more quickly compensate for process line product changeovers or other variations in system components.
Additional advantages and other novel features of the invention will be set forth in part in the description that follows and in part will become apparent to those skilled in the art upon examination of the following or may be learned with the practice of the invention.
To achieve the foregoing and other advantages, and in accordance with one aspect of the present invention, an improved computerized process control system is provided that includes a process equipment line having at least one moving element that moves with a time periodicity that may vary for successive cycles of movement, which is used to produce a material which, for at least one stage of production, forms a moving web of material that is proximal to one or more of the moving element(s); a memory circuit for storing information, at least one input device that measures at least one process parameter of the moving web of material and associates that measurement with its multidimensional position on the web, and a processing circuit that controls a flow of information between the memory circuit and the input device(s); a sensor to determine the position of the periodic element; the processing circuit is configured to receive data from the input device(s) by measuring and numerically quantifying the at least one process parameter during multiple cycles of movement of the at least one moving element(s) and store the numerically quantified information in the memory circuit, then to build at least one Correction Map containing the numerically quantified information acquired over a predetermined time interval and to store the Correction Map in the memory circuit; and the processing circuit is further configured to, during a manufacturing operation, (i) again measure and numerically quantify the process parameter in substantially real time, (ii) determine positions of the moving element(s), (iii) apply the Correction Map(s) to the process parameter that is measured in substantially real time to generate at least one decoupled sample of the process parameter and store the decoupled sample(s) in the memory circuit; (iv) and utilize the decoupled sample(s) of the process parameter to operate the process equipment line, thereby providing a substantially real time measurement correction of product quality variability in the process equipment line.
In accordance with another aspect of the present invention, a method for substantially real time measurement correction of product quality variability in a process equipment line is provided, in which the method comprises: providing a process equipment line having at least one moving element that moves with a time periodicity that may vary for successive cycles of movement, and providing a moving quantity of material that is produced using the at least one moving element(s); providing a processing circuit, a memory circuit to store data, and at least one input device that measures at least one process parameter of the moving material; during multiple cycles of movement of the moving element(s), measuring and numerically quantifying the process parameter, then building at least one Correction Map containing the numerically quantified information acquired over a predetermined time interval; during a manufacturing operation, (i) again measuring and numerically quantifying the at least one process parameter in substantially real time, (ii) determining positions of the moving element(s), (iii) applying the Correction Map(s) to the process parameter that is measured in substantially real time to generate at least one decoupled sample of the process parameter; (iv) and utilizing the decoupled sample(s) of the process parameter to operate the process equipment line.
In accordance with a further aspect of the present invention, a computerized method for building a Correction Map for use in a system is provided including at least one rotating element having an associated moving web of material, in which the method comprises: sampling physical data by use of at least one input device that measures at least one process parameter of the moving web of material; determining a rotational position of each sample for each of the rotating element(s) that is/are being monitored; determining a cross-directional position of each sample of the physical data; updating a Sum Map and a Count Map for each of the rotating element(s), using the sampled physical data; determining if a sufficient amount of sampled physical data has been acquired for convergence to an appropriately small tolerance and, if NO sampling further physical data, or if YES building an Average Map for each of the at least one rotating element(s); and building a Correction Map for each of the rotating element(s), the Correction Map containing a plurality of numeric values related to irregularities in the rotating element(s).
In accordance with a still further aspect of the present invention, a computerized method for controlling in substantially real time a system is provided including at least one rotating element having an associated moving web of material, in which the method comprises: providing a control system for controlling the rotating element(s); providing a Correction Map for each. of the rotating element(s); acquiring a plurality of physical data samples by use of at least one input device that measures at least one process parameter of the moving web of material; determining rotational positions for each of the rotating element(s) that is/are being monitored, for each of the plurality of physical data samples; determining a cross-directional position of each sample of the physical data; by use of the Correction Map, looking up a correction value for each of the rotating element(s); generating rotationally decoupled values in substantially real time by applying the correction value to one of the plurality of physical data samples; and applying the rotationally decoupled values to the control system.
In accordance with yet another aspect of the present invention, a method for correction of product quality variability of a moving web of material in a process equipment line is provided, in which the method comprises: providing a process equipment line having at least one moving element that moves with a time periodicity that may vary for successive cycles of movement, and providing a moving web of material that is produced using the moving element(s); providing a processing circuit, a memory circuit, and at least one input device that measures at least one process parameter of the moving web of material; during multiple cycles of movement of the moving element(s), measuring and numerically quantifying the process parameter, then building a Correction Map containing the numerically quantified information acquired over a predetermined time interval; determining, for each web cross direction position, a correction for non-uniformity in a Count Map, for all of the moving element(s); creating a Correction Profile by summing, for each web cross direction position, the corrections for all of the moving element(s); creating a Measured Profile by averaging samples of the numerically quantified information taken at each cross direction position over the predetermined time interval; creating a True Profile by subtracting the Correction Profile from the Measured Profile; determining a True Machine Direction Average by adding weighted cross direction positions of the True Profile; and controlling the process equipment line using the True Profile and the True Machine Direction Average as corrected values.
Still other advantages of the present invention will become apparent to those skilled in this art from the following description and drawings wherein there is described and shown a preferred embodiment of this invention in one of the best modes contemplated for carrying out the invention. As will be realized, the invention is capable of other different embodiments, and its several details are capable of modification in various, obvious aspects all without departing from the invention. Accordingly, the drawings and descriptions will be regarded as illustrative in nature and not as restrictive.